Help! Computer isn't working!!

Hello again - I know I made a thread about this earlier, but I have tested some components and I think it might be my PSU that's blown. I have an ASUS P4S800-MX motherboard with a Pentium 4 3ghz processor.

When I push the on button on my computer, the fans in the power supply and CPU go, and the light on the motherboard lights up, but the front power button doesn't light up anymore. I also don't get any picture on my screen - it just says "No signal" and the light turns amber. The computer also doesn't beep, so it obviously isn't successfully going through POST.

My DVD burner can open and close, but my other DVD drive won't even open. Could this be due to a faulty power supply or motherboard?

I have resetted the CMOS/BIOS - no luck. I have also checked the RAM to see if it's in properly - it is in properly. I don't know what to do!!

I don't think it's my motherboard though, because some things like my CD-Rom drives and CPU are receiving power fine, but some other things just aren't receiving any power.

Oh I forgot to mention that it isn't my monitor - I've tested my computer with two others and the same thing happens to them, and it also isn't my graphics card I don't think, because I took that out and plugged my monitor into the normal VGA slot and it still didn't work.

If you normally checked your cpu temp from cmos or ran a utility telling you the cpu temp then you can rule out overheated cpu.

If you have not checked the cpu temp I would pull the cpu fan and heatsink and check the thermal grease. Just to make sure there is enough to cover the cpu without enough to run out the sides and possibly cause other problems. I would clean it while you are in there and look for scorching of the cpu just in case.

My computer is about 6 or 7 months old and my brother-in-law helped me put it all together. I'm not exactly an expert on computers, so I don't know if I had a CPU temp monitor - I think I saw it in the motherboard manual somewhere but I don't think I installed it.

Also, what is thermal grease? Is it just gunk that's supposed to be on the CPU? And if there's too little (I don't think there'd be too much though, because I haven't touched it since I bought it) what do I do with it? And how do I clean it?